1. Field of the Invention
The invention relates to a vector modulator, and more particularly relates to a low current vector modulator for continuously varying the amplitude and polarity (i.e., phase) of a radio frequency (RF) signal.
1. Description of the Prior Art
Conventional vector modulators employing PIN diodes are well known in the art of RF signal control. One example of a vector modulator of the prior art is disclosed in commonly owned U.S. Pat. No. 4,016,516 to Sauter et al., the disclosure of which is incorporated herein by reference. U.S. Pat. No. 4,016,516 discloses a solid state signal controller (i.e., vector modulator) designed for insertion in a radio frequency transmission line or path between a source and a signal utilization device to allow control by external means of signal amplitude and polarity, with a minimum of distortion to the signal. PIN diodes or like devices are used within the signal controller.
FIG. 1 shows a conventional vector modulator 10 employing PIN diodes. The vector modulator includes a power splitter 12 having a resistive network 14 for receiving an RF input signal at an input port 16, and three quadrature hybrids 24, 28, 80. Power splitter 12 splits the received RF signal and generates therefrom first and second output signals which are respectively output via ports 18 and 20. An input port 22 of a first hybrid coupler 24 and an input port 26 of a second hybrid coupler 28 receive the first and second output signals, respectively. First hybrid coupler 24 is also provided with an output port 30, a 0.degree. phase port 32 and a 90.degree. phase port 34. Second hybrid coupler 28 is also provided with an output port 36, a 0.degree. phase port 38 and a 90.degree. phase port 40.
A PIN diode 50 is connected at its anode end through a coupling capacitor 52 to 0.degree. phase port 32 of first hybrid coupler 24. A cathode end of PIN diode 50 is connected to ground. A PIN diode 54 is connected at its anode end through a coupling capacitor 56 to 90.degree. phase port 34 of first hybrid coupler 24. A cathode end of PIN diode 54 is connected to ground. Likewise, a third and a fourth PIN diode 60, 62, are connected at each anode end, respectively, through coupling capacitors 64, 66 to 0.degree. phase port 38 and 90.degree. phase port 40 of second hybrid coupler 28, respectively. Cathode ends of PIN diodes 60, 62 are electrically connected to ground.
Common bias is applied to the anode ends of PIN diodes 50, 54 through biasing means 70. Likewise, common bias is applied to the anode ends of PIN diodes 60, 62 through biasing means 72. Biasing means 70 and 72 may be a circuit which provides a selectable voltage to the PIN diodes.
PIN diodes are electrical devices which display a change in resistance with a change in bias current through the PIN diodes. PIN diodes 50, 54, 60 and 62 each display similar electrical characteristics. Biasing means 70 varies the current through diodes 50 and 54 thereby controlling their respective diode resistances. Biasing means 72 varies the current through diodes 60 and 62 thereby controlling their respective diode resistances.
Typical resistance versus current characteristics of a PIN diode, e.g., a UM9301 manufactured by Unitrode Corporation of Massachusetts, are given in FIG. 2. The resistance of the UM9301 PIN diode is inversely proportional to the biasing current through it, as shown in FIG. 2.
Each of first and second hybrid couplers 24 and 28, combined with PIN diode pairs 50, 54 and 60, 62, respectively, act as biphase variable attenuators. Each biphase variable attenuator is able to provide two transmission phase states, one with 0.degree. of reference phase and the other with 180.degree. of reference phase. In each phase state, attenuation may be varied by varying the current through the PIN diode via biasing means 70 and 72 coupled thereto.
The 0.degree. phase state occurs at the output port of the couplers when the resistance of the associated PIN diodes is greater than the characteristic impedance of the system, i.e., the characteristic impedance of the hybrid coupler. The characteristic impedance is typically 50 ohms. The 180.degree. phase state occurs when the PIN diode resistance is less than the characteristic impedance of the hybrid coupler, i.e., less than 50 ohms.
An output signal is electrically coupled from output port 30 of first hybrid coupler 24 to an input port 82 of a third hybrid coupler 80. Output port 36 of second hybrid coupler 28 is electrically connected to an input port 84 of third hybrid coupler 80. A 0.degree. phase (output) port 86 of third hybrid coupler 80 is shown electrically connected to a 50 ohm load 92. A 90.degree. phase (output) port 86 of third hybrid coupler 80 is shown connected to a 50 ohm termination 90. The characteristic impedance of third hybrid coupler 80 is equal to the characteristic impedance of each of first and second hybrid couplers 24 and 28.
Operation of the biphase attenuator function of the conventional vector modulator 10 is as follows.
An RF signal received at input port 16 is split within power splitter 12 thereby generating first and second RF output signals that are respectively output via ports 18, 20. Because both first and second hybrid couplers 24 and 28 and their associated PIN diodes are identical, only a detailed operation of second hybrid coupler 28 with PIN diodes 60, 62 will be described.
The second RF output signal is provided to input port 26 of second hybrid coupler 28. The second RF output signal is split within second hybrid coupler 28 into two portions. A first portion of the second RF output signal is directed to 0.degree. phase port 38 (terminated by PIN diode 60) and a second portion is directed to 90.degree. phase port 40 (terminated by PIN diode 62). The second portion is delayed 90.degree. in phase relative to the first portion. Part of each portion directed to each phase port is reflected back, the phase and amplitude of the reflected portions being dependent on the impedance seen at each phase port, i.e., the resistance of associated PIN diodes 60 and 62.
It is well known in transmission line theory that a high impedance termination produces a reflection coefficient R.sub.e for each signal arriving at each phase port according to the following equation: EQU R.sub.e =(Z.sub.L -Z.sub.0)/(Z.sub.L +Z.sub.0)
where Z.sub.L is the termination impedance and Z.sub.0 is the characteristic impedance of the coupler. The reflection coefficient is the ratio of reflected to incident signal voltage at the termination. As the termination impedance Z.sub.L approaches .infin., R.sub.e approaches one. As the termination impedance Z.sub.L approaches zero, R.sub.e approaches -1, the negative sign indicating the 180.degree. phase state.
When biasing means 72 is adjusted to supply minimum current to PIN diodes 60, 62, the resistance of the PIN diodes is extremely high (see FIG. 2). Because of the high resistive state of PIN diodes 60 and 62, the reflection coefficients at phase ports 38 and 40 of second hybrid coupler 28 approach one. The first and second portions of the second RF output signal are therefore reflected back into hybrid coupler 28 in-phase (i.e., at a 0.degree. phase state).
The first portion is split into two approximately equal signal components when it is reflected from 0.degree. phase port 38. A first component is directed to input port 26 and a second component is directed to output port 36. The second component is delayed 90.degree. relative to the first component.
The second portion of the second RF output signal is also split into two approximately equal components when it is reflected from 90.degree. phase port 40. A first component is directed to input port 26 and a second component is directed to output port 36. The first component is delayed 90.degree. relative to the second component.
Therefore, the first component reflected from 90.degree. phase port 40 arrives at input port 26 delayed twice by 90.degree. relative to the first component arriving there from 0.degree. phase port 38. The superposition of those two components cancel. The second components reflected from 0.degree. phase port 38 and 90.degree. phase port 40 towards output port 36 arrive thereat in phase since each has been delayed once by 90.degree.. These components add constructively. Thus, an RF signal received at input port 26 of second hybrid coupler 28 (when PIN diodes 60 and 62 are biased to a resistance greater than 50 ohms) appears at output port 36 substantially unchanged in amplitude and 90.degree. out of phase with respect to the signal at the input port 26.
Alternatively, if the 0.degree. phase port 38 and the 90.degree. phase port 40 of second hybrid coupler 28 are short circuited to ground, reflection coefficient R.sub.e at each of those ports approaches -1. By providing high current through PIN diodes 60, 62 by biasing means 72, the resistance of the PIN diodes decreases towards zero ohms. The low resistive state at the 0.degree. and 90.degree. phase output ports 38 and 40, respectively, causes first and second portions of the second RF output signal to be reflected back into hybrid coupler 180.degree. out of phase (i.e., at a 180.degree. phase state).
The superposition of the first signal components reflected from 0.degree. and 90.degree. phase ports 38 and 40, respectively, and arriving at input port 26 results in cancellation there. The superposition of second signal components reflected from the 0.degree. and 90.degree. phase ports, respectively, and arriving at output port 36 results in constructive addition there. Thus, the input signal provided to port 26 appears at output port 36 substantially unchanged in amplitude. The phase, however, is shifted 180.degree. relative to the previously described case in which the PIN diodes are biased to a high resistance state. That is, the output signal at output port 36 while PIN diodes 60 and 62 are biased with a high current displays a phase of -90.degree. relative to the phase of the signal input to the hybrid coupler.
When PIN diodes 60 and 62 are biased to resistance values between a minimum and 50 ohms (the characteristic impedance of the second hybrid coupler), the reflection coefficient at both the 0.degree. and 90.degree. phase ports 38 and 40 varies between approximately -1 and 0. Consequently, the magnitude of the signal appearing at output port 36 may be varied from its approximate maximum value to around zero at the 180.degree. phase state.
When PIN diodes 60 and 62 are biased to resistance values between 50 ohms and their maximum, unbiased impedance, the reflection coefficient at both the 0.degree. and 90.degree. phase ports 38 and 40 varies between 0 and 1. Consequently, the magnitude of the signal appearing at output port 36 may be varied between 0 and approximately 100 percent of its maximum value at the 0.degree. phase state.
Thus, the arrangement of a hybrid coupler with biasable PIN diodes in vector modulator 10 provides biphase, variable attenuation to the second RF output signal from second hybrid coupler 28. The phase of the resulting signal at output port 36 is wholly dependent on the resistance of PIN diodes 60 and 62. If the resistance of PIN diodes 60, 62 is greater than the characteristic impedance of hybrid coupler 28, i.e., 50 ohms, the phase of the signal provided at output port 36 is 90.degree.. If the resistance of PIN diodes 60, 62 is less than 50 ohms, the phase of the signal at output port 36 is -90.degree..
First hybrid coupler 24 (including PIN diodes 50, 54) operates in a manner similar to that of second hybrid coupler 28 and PIN diodes 60 and 62. Therefore, a first component of a first portion of the first RF output signal reflected from 0.degree. phase port 32 arrives at output port 30 in phase with a first component of a second portion of the first RF output signal reflected from 90.degree. phase port 34, adding constructively. A second component of the first portion of the first RF output signal reflected from 0.degree. phase port 32 arrives at input port 22 180.degree. out of phase with a second component of the second portion reflected from 90.degree. phase port 34, whereby the second components cancel. The phase of the signal provided at output port 30 is either 90.degree. or -90.degree., depending on the resistive state of PIN diodes 50, 54.
Output signals from output port 36 of second hybrid coupler 28 and output port 30 of first hybrid coupler 24 are combined in third hybrid coupler 80. Third hybrid coupler 80 provides the combined signal to load 92. The specific phase angle and amplitude of the output signal from the vector modulator depend on the amount of attenuation provided by the PIN diodes operating in conjunction with each of first and second hybrid couplers 24 and 28.
There are, however, problems associated with the use of conventional vector modulators, especially in situations where low power consumption is a constraint. Since conventional vector modulators employ hybrid couplers with a characteristic impedance of 50 ohms, the PIN diodes must be driven with sufficient current so that their intrinsic impedance matches the 50 ohm characteristic impedance of the couplers and or as close to 0 ohms as possible when minimum loss with 180.degree. phase state is desired. The minimum resistance is limited by the amount of current available. FIG. 2 shows this current as being about 2 mA per diode. Although such a current may not seem excessive in typical situations, there may arise situations where even lower current drain and power consumption by the vector modulator are required.